The present invention relates to a receiving apparatus for an OFDM communication system used in mobile communication or the like. More particular, this invention relates to a receiving apparatus for performing timing synchronization on OFDM transmission by using specific known patterns included in a reception signal.
A conventional receiving apparatus for an OFDM communication system will be explained below. For example, in wide-band mobile communication, arrival timing of a signal dynamically changes due to changes in the distances or the multi-path. In particular, since multi-carrier communication represented by OFDM (Orthogonal Frequency-Division Multiplexing) is sensitive to an error of reception timing, it is necessary to do some amends.
As literature related to the receiving apparatus for the OFDM communication system which performs some kind of compensations, for example, xe2x80x9cA High Performance Frequency and Timing Synchronization Technique for OFDM, THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS, TECHNICAL REPORT OF IEICE., SSE98-21, RCS 98-21 (1998-04)xe2x80x9d is known. The conventional art will be explained with reference to this literature. FIG. 7 is a block diagram showing the configuration of a receiving apparatus in a conventional mobile radio communication system. FIG. 8 shows a burst format of a conventional OFDM signal.
A reception signal (OFDM signal) S11 received by a receiver is input into a correlation calculation unit 14. In the reception signal S11, as shown in FIG. 8, subsequent to preamble written in a start portion of the burst, specific patterns each having a length B indicated by xe2x80x9cpattern Axe2x80x9d are repeatedly written (FIG. 8 shows a case in which the patterns are continuously written for two times). In the correlation calculation unit 14 into which the reception signal is input, the correlation value of the section of the continuous lengths B is calculated while shifting the calculation position. More specifically, while the calculation position is shifted at every predetermined time, correlation between the earlier xe2x80x9cpattern Axe2x80x9d and the later xe2x80x9cpattern Axe2x80x9d is taken a plurality of times, and correlation value distribution information S15 is output.
In a maximum correlation value searching unit 15, a position indicating the maximum correlation value is selected from the obtained correlation value distribution information S15, and maximum correlation value position information S16 is output. A timing adjustment unit 11 receives the reception signal S11 and the maximum correlation value position information S16. This timing adjustment unit 11 determines a start position of a data portion in the reception signal S11 based on the reception signal S11 and the maximum correlation value position information S16, and outputs a data signal S12 constituted by only the data portion to a time/frequency conversion unit 12.
Since the OFDM signal, i.e., the data signal S12 is multiplexed such that information is dispersed to a plurality of sub-carriers, the time/frequency conversion unit 12 converts the received data signal S12 from a signal on a time axis into a signal on a frequency axis, and information S13 on each sub-carrier is extracted. This extraction is performed using the FFT (Fast Fourier Transform).
Finally, the information S13 converted into data on each sub-carrier is transmitted to a detection unit 13. The detection unit 13 detects the information S13 and demodulates the information S13 to output reception information S14.
As described above, in the conventional receiving apparatus for an OFDM communication system, correlation between continuous repetitive patterns is used to detect a synchronization position, and a position where a reception signal having the maximum power is input in an ideal state being free from noise, i.e., a position indicating the maximum correlation value is estimated as the synchronization position.
However, in the conventional receiving apparatus for an OFDM communication system, since correlation between continuous repetitive patterns is used to detect a synchronization position, for example, as shown in FIG. 9, portions C in FIG. 9 are the same as each other, and portions D and E are different from each other. Thus, a difference between correlation values sometimes becomes very small. Therefore, a method of estimating a synchronization position based on correlation value distribution information obtained by the calculation method is very poor at noise in signal reception, and preferable synchronization characteristics cannot be obtained in an actual communication environment in which multi-path fading or the like occurs.
In the conventional receiving apparatus for an OFDM communication system, as described above, a position where a reception signal having the maximum power is input is estimated as a synchronization position in an ideal state being free from noise. However, in an actual communication environment, it is unsure that the power of a preceding wave is always stronger than that of a delay wave. In general, in the OFDM communication system, as shown in FIG. 10, a redundant portion called a guard interval is formed ahead of the information portion. In the guard interval, the influence of interference caused by the delay wave is suppressed.
Therefore, although the influence of the delay wave in the guard interval is not very large, a received wave outside the guard interval serves as an interference component to adversely affect the operation of the OFDM communication system. Therefore, as shown in FIG. 10, in a channel having the characteristic that the power of a delay wave is higher than that of a preceding wave, a preceding wave (interference wave component) is arranged ahead of the guard interval, and the preceding wave degrades synchronization characteristics.
The present invention has been made in light of the problems described above. It is an object of the present invention to obtain a receiving apparatus which can obtain preferable synchronization characteristics even in a communication environment having high noise power.
According to one aspect of the present invention, unlike the conventional art where a correlation between continuous repetitive patterns is used to detect a synchronization position, the correlation between the continuous repetitive patterns is not used but signal patterns identical to specific known patterns are calculated. A calculation result is prepared in the apparatus in advance, and correlation calculation between a reception signal and the signal patterns is performed while shifting the position of the correlation calculation for every specific time. Further, a correlation value calculation unit for forming signal patterns identical to the known patterns in such a manner that an OFDM signal is converted from a frequency axis signal to a time axis signal is provided.
According to another aspect of the present invention, unlike the conventional art, a position where a reception signal having a maximum power (maximum correlation value) is input is not estimated as a synchronization position. For example, a intensity distribution of reception signals is calculated based on the plurality of obtained correlation values, a synchronization position where the power value of the reception signal arranged out of a guard interval is minimum is determined, and the synchronization position is set as a reference position of timing synchronization. In this manner, the maximum signal/interference wave power ratio can be realized for the reception wave, and preferable synchronization characteristics can be always obtained.
According to still another aspect of the present invention, unlike the conventional art where correlation between continuous repetitive patterns is used to detect a synchronization position, the correlation between the continuous repetitive patterns is not used but signal patterns identical to known specific multi-carrier time waveform pattern are calculated. A calculation result is prepared in the apparatus in advance, and correlation calculation between a multi-carrier time waveform pattern and the signal patterns is performed while shifting the position of the correlation calculation for every specific time. Further, a correlation value calculation unit for forming signal patterns identical to the multi-carrier time waveform pattern in such a manner that an OFDM signal is converted from a frequency axis signal to a time axis signal is provided.